texts: Expansion of space - how do we end up with an unobservable region?

texts:
I've just been answering a question on this issue,in physics SE,and realised that my answer conceals a point I don't myself understand.

I understand that

The expansion of metric isn't affected by the SR speed of light/signal propagation, and that this resulted in objects placed at separations (as measured within spacetime) where they can't be detected or signals received from them in the 13.7 bn years since the Big Bang.
The rough scale of expansion discussed is ~ 10^26 linearly (roughly 1nm -> 10 ly) and most of that took place in a tiny period of time, at about 10^-32 or so seconds, continuing much slower since then, and speeding up again in the last while.
The expansion is theorised to be triggered by energies/effects of a phase change or decomposition of a unified field, meaning it didn't happen at all until it did. One implication seems to be that any expansion increased distance between objects in space which might have arisen before expansion, as modelled by current theories, would have been a result of some physical movement affecting the distribution of energy/transient particles/whatever existed, within spacetime, and was not due to any earlier episode of metric expansion.

So far so good.

What I don't get is this:

Suppose we model the dominant modes of expansion of the universe and separation of objects within it, like this:

physical changes to distribution within spacetime (objects move apart within space) for first 10^-32 sec
metric expansion instantaneously of the order 10^26, at 10^-32 sec or so.
broadly, expansion in space since then - I know its a mix of that and continuing but slower metric expansion, but not sure if that's affecting the line of logic.

Starting at a point at time zero, we find that two objects would be at most ~ 2 x 10^-32 light seconds apart at the start of expansion, or 2 x (3x10^8) x (10^-32) m apart = 6 x 10^-24 m. Therefore after expansion two objects could be at most (6 x 10^-24) x (10^26) apart = 600m apart. That's still a pretty trivial distance for light or signals to propagate, though

(I agree, if objects had initially been 1km or 1 ly apart immediately prior to the expansion epoch, they would now be billions of light years apart after expansion, and unobservable. But by definition, before expansion they just  didn't have time to become that far apart, they could at most become a tiny distance apart, because only ~ 10^-32 sec had elapsed)

So after initial expansion it seems that the universe could have been at most 600m diameter. Similar logic applies to the much slower expansion  since then.

But a non observable universe effect seems to imply that after initial expansion, the universe was light years across, and objects in it were able to be far enough apart to be unobservable now, meaning > 13.7 ly apart. But that would imply a much larger expansion than 10^26, or a much larger size before expansion than there was time to achieve pre-expansion (which would have been predominantly SR limited and within spacetime, hence order of 10^-24m as above).

What is the resolution of this paradox / point I'm missing?


texts: Throughout the year, are all parts of the night sky visible from any given point on Earth?

texts:
Or are there points on Earth where certain parts of the 360° night sky are never visible throughout the whole year?

Depending on the answer to this question, are there points on Earth where parts of the celestial sphere as such are never in the sky, no matter if it's a day or night sky?


texts: How big would the Milky Way be if all the stars were emptied into one sphere?

texts:
Neglecting the fact that this ball of gas would just collapse on itself - I'm curious if there's an agreed-upon measurement that takes into account the volume of all the ~100 billion stars to predict the diameter of a milky way where all the gas is in one orb. If not, is it safe to just use the volume of our Sun (which is an "average" star) and multiply by 100 billion or is there a more accurate data set to include in the calculation?
Asking for a 6-year-old :)


texts: Why are there no gamma-ray bursts detected in our galaxy?

texts: I found from Wikipedia and other sites that there are no GRBs detected in the Milky Way. Can someone give a feasible reason for that? Why are there no GRBs detected in the Milky Way galaxy?


texts: Why would the interplanetary medium atomic hydrogen Lyman‐α emission fall as 1/r rather than1/r^2?

texts:
The Live Science article NASA Spotted a Vast, Glowing 'Hydrogen Wall' at the Edge of Our Solar System links to Geophysical Research Letters The Lyman‐α Sky Background as Observed by New Horizons https://doi.org/10.1029/2018GL078808 the abstract of which is

  Recent observations of interplanetary medium (IPM) atomic hydrogen Lyman‐α (Lyα) emission in the outer solar system, made with the Alice ultraviolet spectrograph on New Horizons (NH), are presented. The observations include regularly spaced great‐circle scans of the sky and pointed observations near the downstream and upstream flow directions of interstellar H atoms. The NH Alice data agree very well with the much earlier Voyager UVS results, after these are reduced by a factor of 2.4 in brightness, in accordance with recent re‐analyses. In particular, the falloff of IPM Lyα brightness in the upstream‐looking direction as a function of spacecraft distance from the Sun is well‐matched by an expected 1/r dependence, but with an added constant brightness of ~40 Rayleighs. This additional brightness is a possible signature of the hydrogen wall at the heliopause or of a more distant background. Ongoing observations are planned at a cadence of roughly twice per year.

The constant brightness (indicator of "the wall") makes sense to me, a large resolved surface of constant emission intensity (W/m^2) will have a constant brightness, independent of distance.

Conversely, a point source's brightness will obey a $\frac{1}{r^2}$ (inverse square) law.

If absorption were the reason for the fall off, it would be Beer-Lambert as $e^{-\alpha r}$.

But the abstract describes a 1/r fall off of the interplanetary medium (IPM) atomic hydrogen Lyman‐α (Lyα) emission.

Question: How does a $\frac{1}{r}$ falloff arise in this case?


texts: How to calculate exact angle to the moon?

texts:
I am building some device as a gift for my girlfriend.

For this device to work, I need to be able to calculate the angle to which I should look towards (3D angle), in order to see the moon.

This angle is affected by (and maybe not only):

The time of year.
The hour.
The location on earth from which I look.

I'm sure there's some formula that approximates the angle.

I don't need an exact value, I can live with +-1 degree.


texts: Available energy for potential life on Titan

texts:
There is an interesting discussion about whether there could be life on Saturn's moon Titan. (For example here.) The life could use reaction of hydrogen and acetylene which are being produced by photolysis in Titan's atmosphere. I would like to know what total energy is available for such life per square meter per day, or at least how much hydrogen is produced in Titan's atmosphere per day.

There are some estimates in article McKay and Smith, Icarus, 2005, but I have problem translating the quantities they use for the estimate to total amount of hydrogen produced per unit of time or energy produced per unit of time.


texts: Why can't we build a huge stationary optical telescope inside a depression similar to the FAST?

texts:
So first we'll have to find a natural depression or we should create one. I understand you can only see a single portion of the sky since it can't be moved, so my money is on creating an artificial basin in an area where directly upwards it has the best view of the sky.
Imagine if we can build a 1000 m optical telescope like that. We'd be able to see exo-planets directly. We'd be able to see ancient galaxies. Even a 200 m one like this would do wonders. Are there any proposals?


texts: How to listen to our world from 10,000 light years away?

texts: If you were 10,000 light years away from all radio stations and digital broadcasts from our world, what equipment would be necessary to detect and capture some information like a voice or a photo? If we found a similar planet in the galaxy, what could we do to listen to their radios?


texts: Which techniques are used to convert radio signals received by antenna to images?

texts:
I realized that after answering the question How do astronomers convert radio signals received by their antenna to images? I had missed the point that the OP was only asking about software.

I've written this question about experimental techniques as a companion question in order to move my answer from there to here.


texts: Interfacing FORTRAN programs for model atmospheres with IDL or Python

texts:
I'm trying to use model atmospheres such as Kurucz or MARCS for the first time personally and I'm wondering if it is better to wrap it with IDL or Python in terms of writing the code, and getting the values I wanted from the model atmospheres into arrays for printing into tables.

Essentially is it better to approach this issue using IDL or with Python, or is it just more of a preferential thing if anything?


texts: A black hole that doesn't take in matter?

texts:
According to List of Common Misconceptions by Wikipedia, under the heading of astronomy, this line can be found:

  A black hole can act like a "cosmic vacuum cleaner" and pull a
  substantial inflow of matter, but only if the star it forms from is
  already having a similar effect on surrounding matter.

This is referenced to a paper written in Yale university:  "Frontiers And Controversies In Astrophysics Transcript 9", although that's a dead link. By linking so, I believe that it is a reliable source.

This got me thinking that is there a black hole that doesn't suck in surrounding matter? And is there a star that don't take in surrounding matter?


texts:
I have just started studying Cosmology and we have been asked to prove that in an expanding FRW Universe which obeys the strong energy condition: $$\rho + 3P >0$$
Then there must exist a Big Bang singularity. I can see that this condition implies $$\ddot{a}/a \leq 0$$
which when you plot against t gives an always decreasing rate of expansion over time. This when plotted then shows that if you extend the curve far back enough will cross the t axis at some finite t-value representing a Big Bang singularity. My question is why can't the universe begin with some finite non-zero a value? is there some physical reason that we expect the Universe to begin with a=0?


texts:
It’s well known that (lunar) tides on Earth result in a transfer of angular momentum from Earth proper to the Earth–Moon orbital motion. That’s why the Moon resides now in a high Earth orbit, and Earth’s axial rotation is nowadays much slower than in the past (Hadean, Archean etc.).

What about effects of solar tides, indeed? The angular momentum certainly must be transferred from Earth proper to the Earth–Sun orbital motion. Does it result in a tiny increase of (Earth+Moon)’s heliocentric orbital angular momentum over millenia? What about Mars and Ceres: does the solar tide brake their axial rotation and change slightly their orbits? And what about Venus: does its solar tidal braking explain the slowness of rotation completely?

Addendum. The solar tide may not be confused with “dual” effect: planetary tide on the central star. It exists as well and has much significance for (extrasolar) planets on near orbits — but this question is not about tides on Sun. Ī̲’d accept an answer assuming gravity of the Sun to be spherically symmetric, neglecting Sun’s deviation from spherical symmetry, as well as rotation (even going to account for General Relativity). The solar tide is interaction between this gravity and the orbiting object. By Newton’s laws, gravitation affects Sun’s momentum reciprocally (and, hence, velocity), and we may ignore here any effect on Sun beyond its translational motion (momentum/velocity/position).


texts:
I visited:

http://aa.usno.navy.mil/imagery/moon

http://www.usno.navy.mil/USNO/time/moon-phase-images

within seconds of each other, and saw two very different "current moon images":

Is one of these images wrong? The larger image seems to display a much
bigger crescent than the smaller image.

I've noticed this earlier as well.


texts: Which telescope to choose and some general questions

texts:
I'm playing with the idea to buy a telescope for years now and finally decided to do so. There are two which I consider:

The Celestron NexStar 127 SLT Mak and the Celestron NexStar 130 SLT

I already know the difference between a Refractor and Maksutov-Cassegrain and for which observations they're suited for best (and also that the Mak doesn't need collimating).

First question: Which would be the better choice for a beginner and eventually astrophotography?

Second question: Is it possible to buy just the tube later (so I can reuse the motors and the tripod)?

Third question: As I've read, the tripod is pretty wobbly, so is it possible to replace the tripod later on (or is there a norm for the mounts)?


texts: Which approximations are required for the synodic period formula?

texts:
The formula below is used to calculate the synodic period between two bodies orbiting a same third one:

 $$\dfrac{1}{T_\mathrm{syn}} = \dfrac{1}{T_{1}} - \dfrac{1}{T_{2}}$$

This formula can be used, for instance, to deduce the period of a planet in the Solar System, given that $T_\mathrm{syn}$ can be measured (period between two consecutive observations of a planet in the same relative position in the sky) and taking $T_1$ as 365.26 days.

I have read that this formula is derived from:

$$w_{\mathrm{syn}} = w_{1} - w_{2} = \dfrac{2\pi}{T_\mathrm{syn}} = \dfrac{2\pi}{T_{1}} - \dfrac{2\pi}{T_{2}} \therefore \dfrac{1}{T_\mathrm{syn}} = \dfrac{1}{T_{1}} - \dfrac{1}{T_{2}}$$

My question is: which approximations are required for this result to hold true? Must the orbits be circular? Must these two bodies be in the same plane around the third? Also, considering the case of elliptical orbits but still assuming that the mass of the third body $M ≫ m_{1}$ and $M ≫ m_{2}$, would the synodic period of body 2 as observed from body 1 be indeed constant?


texts: How the observed deuterium abundance constrains big bang nucleosynthesis?

texts: I know deuterium can be used as a good tracer to constrain the big bang nucleosynthesis. But can anyone tell me the in details how this is actually done?


texts: How can an accretion disk form around the central star of a planetary nebula?

texts:
I was reading the 2012 paper A tidally destructed massive planet as the progenitor of the two light planets around the sdB star KIC 05807616 about the possible origin of two exoplanets that orbit a subdwarf B star. The researchers suggested that they could have been formed by the fragmentation of the metal core of a gas giant that was orbiting the star while it was still in the main sequence. Just at the moment when it became a red giant and absorbed the planet, which was fragmented by tidal forces when it reached the core.

Here is an excerpt from the summary:

  We suggest the following alternative scenario for the formation of the two planets (section 2). A single massive planet of mass $m_P \gtrsim 5M_J$ went through the CE evolution inside the RGB envelope. It spiralled all the way to the center. The released gravitational energy is behind the removal of the stellar envelope, as is commonly the case with CE evolution. The massive planet reached the tidal destruction radius (eq. 1). The gaseous mass of the planet was lost and part of it formed a temporary accretion disk around the core, that now is the EHB star. The metallic core of the massive planet was tidally destructed into several Earth-like bodies immediately after the gaseous envelope was removed. Different bodies had different energy per unit mass. Some of them spiral-in and were further destructed by the core, while other survived at orbital separations of $\gtrsim 1R_⊙$ within the gaseous disk. The bodies interacted with the disk and among themselves and migrated, as planets around young stars do. Two of the bodies survived and reached an almost resonance. These are the observed Earth-like planets.

I infer that at the time the gaseous accretion disk was formed around the core, the planetary nebula was still in formation, since the high temperatures of the core must have ionized the surrounding gas, at least for a few thousand years.

I understand that the gas that is expelled from the interior of a dying star in its red giant phase does so at several kilometers per second. My question is: why the gaseous accretion disk that formed around the core was not immediately ejected into space as was the case with the gas from the nascent planetary nebula? Why was it gravitationally tied to the core while the surrounding gas was escaping from gravity?


texts: Ideas for a project in astronomy and deep learning

texts: I am currently pursuing computer engineering and am in my senior year. I'm looking for a long term project (ideally 2-4 months) in astronomy and deep learning. I have extensively worked on the Galaxy Zoo project and am pretty content with the results. I'm looking for similar projects with easily available datasets. Can someone suggest me some ideas?


texts: What parameters determine whether galaxies colliding will result in a merger or a hit and run?

texts:
Colliding galaxies sometimes merge and sometimes pass through each other. Either way, there are huge changes as a result. What are the parameters that matter in determining whether a collision will result in a merger or a hit and run, or something in between? What will be the result of the Milky Way-Andromeda collision?

Thinking about this some more, I suppose the most clearcut distinctions for the result would be whether the two supermassive black holes end up unbound, bound, or merged. Would it be possible for the bulk of the two galaxies to merge, but one or two of the SMBH's go on their merry way?


texts: Determining the percentage area of the moon's disk shaded with respect to time

texts:
I've looked online and I've found sources such as this paper on a lunar phase simulator as well as this article about lunar phases.
I'd like to be able to model the area of the lunar disk which is shaded with respect to the moon's angle or time of the lunar month and I'm not sure how to go about doing this. In essence, I would like to be able to find a function f(x) where I can feed in a value such as 0 (meaning 0 days since the start of the lunar month) and I'd be able to get an output like 0 since it's a new moon. Likewise, f(14ish) should be new moon or fig 5 on the image below and so I'd get a value close to 1.
I'm focused a bit more on the maths rather than real values, so I'm willing to make assumptions such as the moon and earth being perfect spheres for the sake of the model.

I know there's a lot of maths involved and I'm going to have to do some integration, but I'd appreciate it if someone could point me to sources to get started on this since the papers I'm finding are either children's lunar phase worksheets or not related to this topic.


texts: Life planets orbiting black-holes. Can/Do they really exist?

texts:
So, I watched Interstellar and if you watched it too you know that there's a planet orbiting a black-hole, they call it Miller's Planet. According to the movie, every hour on Miller's Planet is equivalent to 7 years on Earth due to gravitational pull from the black-hole.

Question: Assuming there are other life forms in the universe, is it really possible for it to be near a black-hole? Is it possible that they came to existence thousands (or even millions) of years before us, but are not as advanced as us because time on our planet is way quicker than that of theirs? is a lot more for us? If they have an assignment due tomorrow, we have, say, 100 years more time to do it (assume what else we can do in that 100 years). Or they actually are more advanced than us, but from Earth, they somehow live in the past?


texts: nonagesimal in Kepler's parallax computation

texts:
I'm starting to learn Kepler's Astronomia Nova using William Donahue's translation . In the translator's introduction, it is demonstrated how Kepler did his parallax computation. I failed to understand the following paragraph:

specifically:

1) nonagesimal's definition: there are 2 points of intersection between the ecliptic and the horizon: one in the East, the other in West. another ambiguity is the direction of the $90^\circ$

2) I can't see why the altitude of the nonagesimal is "the same as the angle between the ecliptic and the horizon". It does make sense to me, but I'm missing a kind of "proof" for that.

3) Actually I also failed to understand what follows in that paragraph - but maybe your explanation for the points above, will clarify that for me.

your help is very much appreciated


texts:
As mentioned on the NASA eclipse website the eclipse on the 3rd of November 2013 will be a "Hybrid" eclipse.

I have looked at the diagram on that page but can't see what is different about the eclipse that would explain the word hybrid. This terminology also confuses me:

  It is one of the rare hybrid or annular/total eclipses in which some sections of the path are annular while other parts are total.


texts: Why the torque exerted by the moon upon the Earth makes the Moon to increase his orbit?

texts: For what I know, this torque exists because of the misalignment of the tidal bulge with the apsidal line Earth-Moon a certain angle $\alpha$, which makes the earth rotation to slow down a bit. But, how that pulls away the moon? I thought this "loss" of rotational momentum transferred into tidal heating. I fail to see how this energy is transferred into more orbital momentum, because I thought the conservation of angular momentum didn't have to do with the rotation of the objects. I mean, isn't true that $c$ remains constant in the two-body problem (being $c = \vec{r}\times \dot{\vec{r}}$)? How the rotation plays into that?


texts: Compute the total stellar mass of galaxy with major influence of Red Giants luminosity

texts:
Suppose that all stars in this galaxy were born in a single major-merger burst event about 10 Gyr ago

If the luminosity in the B band (absolute magnitude in B-band is equal to -21.22) is dominated by stars of in the RG branch, with masses $m \sim 1\,\text{L}_{\odot}$ (within $\sim 10\%$) and average luminosities $\sim 1000\,\text{L}_{\odot}$.

How can I estimate the total stellar mass of this galaxy using the Schechter relation ?

I think that I have to use the definition of Schechter law:

$$N(L)\ \mathrm {d} L=\phi^{*}\left({\frac{L}{L^{*}}}\right)^{\alpha}\mathrm {e}^{-L/L^{*}}{\frac{\mathrm {d} L}{L^{*}}}$$

or maybe Salpeter relation : $$\text{d}N=0.06\,\times\,M^{-2.35}\,\text{d}M$$

But how to introduce the parameters of Red-Giants of $1\,\text{M}_{\odot}$ with $L=1000\,\text{L}_{\odot}$.

Initially, I calculate the fraction of masse between $m_{1}=0.9$ and $m_{2}=1.1\,\text{M}_{\odot}$ :

$$\text{d}N(m_{1}<m<m_{2})=\int_{m_{1}}^{m_{2}}\,\Phi(m)\,\text{d}m=0.06\,\dfrac{(0.9^{-1.35}-1.1^{-1.35})}{1.35} = 1.22 \%$$

Anyone could see the trick to compute total stellar mass from these parameters with above laws ?

Regards


texts:
So black holes are created by certain dying stars and when the star runs out of nuclear energy gravity wins out and the star implodes. An entire star’s mass collapses down into a smaller and smaller volume of space. Which then creates a black hole, so my question is, do black holes actually have energy to create that type of vacuum to suck light in?

If a black hole was created by a huge star running out of fuel and then collapsing in on it self, would the black hole have energy and if so where would it come from, or is it just like the vacuum in space with no energy at all?

(Please correct me if I said anything wrong about the black holes as I am very young and don't quiet grasp the concept of it yet.)


texts: Mini black holes, a possible result of the LHC experiment at CERN

texts:
A scientist at CERN LHC (Large Hadron Collider) was asked during an interview about the possible results and consequences of operating the LHC at full power of collision in the near future, one of the results he literally said :

  One of the theories that predicts the consequences of operating the LHC at full power collision is that mini black holes might form, but do not be afraid of those black holes, they will not start expanding and destroy the earth, they will instantaneously disappear.

I know that black holes create disturbance in space-time. And they actually appear when matter massively gets pressurized into a small size of volume in space which causes gravity to increase insanely which in turn initialize a black hole that even light its self can not escape away from.

So how can a mini black hole disappear anyway?
What in the world would cause a small size, a mini size, a tiny little black hole to vanish or disappear?


texts: As the title states, If earth were the size of a marble would it be smoother than a marble?


texts: Can a dying, swelling, star be rejuvinated by enveloping orbiting gas giants?

texts:
Many hot jupiter type exoplanets have been found, orbiting near their parent stars.

At the end of a stars life, they swell up and sometimes envelop closely orbiting bodies.

Combine these effects and what could/would happen? Could the unfused hydrogen of a gas giant be dragged into the stars core and end up rejuvinating the star and extend its lifetime? Have these conditions been contemplated before? What might be expected to happen? Or is the environment of a stars outer layers just not dense enough to keep a gas giant from just continuing in its orbit?


texts: When was the last Great Conjunction on Winter Solstice?

texts:
The Wikipedia Great Conjunction page only lists the years 1206-2398 and in those 1192 years there is no Great Conjunction landing on the Winter Solstice.
When was the last Winter Solstice Great Conjunction and when will the next one be? I want to know how rare this one is.


texts: Which kind of strategy should we take to discover Planet 9 and other KBOs?

texts:
What are the differences between the discoveries of Sedna, 2012vp 113, and the supposed Planet 9?

All need deep infrared surveys at the ecliptic plane, but P9 needs a much deeper survey?


texts: How can a planet not rotate at all and yet have 1 year that is day and 1 year that is night?

texts:
A mercury day if you consider its solar day is 2 mercury years.

Now if you consider just 1 half of mercury, in 1 of those years it is facing towards the sun and in another one of those years it is facing away from the sun.

Mercury is tidally locked. This tidal locking happens when the rotation rate slows down until it reaches 0.

How is it possible for a planet to not rotate and yet have 1 half of it facing towards the sun each year with that half changing every year? It would seem like it would have to rotate 180 degrees in a fraction of a second which I know is not possible.


texts: Why would tidal forces on planets become more intense when a star becomes a white dwarf?

texts:
I was puzzled by this press release by the Royal Astronomical Society: "Small, hardy planets most likely to survive death of their stars" - phys.org, May 14, 2019

  Astrophysicists from the Warwick Astronomy and Astrophysics Group modeled the chances of different planets being destroyed by tidal forces when their host stars become white dwarfs and have determined the most significant factors that decide whether they avoid destruction.

According to the shell theorem, the gravitational forces of a spherically symmetrical body's regions, in terms of how they act on external objects, sum up to the functional equivalent of a point mass. A star therefore acts like a point gravity source both before and after becoming a white dwarf (albeit a lower-mass one after, due to casting off matter that becomes a planetary nebula — a drop in attraction that I'd expect to cause any remaining planets, those not close enough to have been slowed and swallowed by the red giant phase, to migrate outward).

So why would a star, after turning into a white dwarf, exert stronger tidal forces on its planets?


texts: Duplicate entries in Tycho-2 main and supplement 1

texts:
When checking the internal consistency of the Tycho-2 catalogue, I came across 254 entries in the main catalogue file that are also present in supplement 1 (the Hipparcos/Tycho-1 stars not included in Tycho-2). These entries have the same TYC1/2/3 identifiers, and the same HIP number, but they refer to different CCDM components. For all but a few cases, the A component is found in the main catalog file, while the B component is found in supplement 1.
From the description Tycho-2 catalogue guide, I would expect these stars to have at least a different TYC3 identifier. Does anybody know why these stars were not given a unique TYC identifier?

Brightest example: HIP 84606 (TYC 2604-1775-1)

I loaded the Tycho-2 catalogue into a MySQL database, the following query selects all the duplicate entries, ordered by increasing magnitude:

SELECT tyc, MIN(HIP) AS hip, MIN(VTmag) AS mag FROM
(
    SELECT CONCAT(TYC1, '-', TYC2, '-', TYC3) AS tyc, HIP, VTmag FROM tyc2
    UNION ALL
    SELECT CONCAT(TYC1, '-', TYC2, '-', TYC3) AS tyc, HIP, VTmag FROM suppl_1
    UNION ALL
    SELECT CONCAT(TYC1, '-', TYC2, '-', TYC3) AS tyc, HIP, VTmag FROM suppl_2
) AS t1
GROUP BY tyc HAVING COUNT(tyc)>1 ORDER BY mag ASC


texts: How the watches are showing perfect time?

texts:
The actual time for one day is 23hrs, 56mins, 4.1secs right? Then how can the clocks and watches can show perfect time?
I mean, if I observe the sun rise at 6:00am this day, tomorrow I should observe it before 6:00 or I can say at 5:56 approximately as our measuring devices follow 12+12 i.e., 24hr day. But in practice again it will be 6:00am when the sun rises. How it is possible?


texts: Light distorsion/ilusion when looking at bright objects

texts:
What's the name of the phenomenon when you look at a bright object (e.g. the moon) and see some perpendicular "beams" going top-down and right-left (bare eye observation, also slightly visible on iPhone pic)?

https://ibb.co/j2b4AF
see the link for a picture


texts: Do planetary rings always follow rotational axis?

texts: In solar system,most ring systems seem almost lies exactly on the equator of planet, is it possible that the plane of a ring system is not lie on the equator of the planet?


texts: Why was the (small) Hubble better able to find KBO targets for New Horizons than large adaptive optics ground telescopes?

texts:
When initial searches for a Kuiper Belt object for New Horizons to fly to after passing Pluto did not find good targets, the Hubble telescope was used, and it resulted in the current targeted flyby for 2019. Initial searches used ground based telescopes. When the search risked running out of time without a good target, the Hubble was brought in to help.

According to what you read about the current generation of large earth based telescopes that use adaptive optics, those telescopes have a much larger angular resolution and light gathering area than the Hubble. So why was the Hubble better capable of finding a good target?

edit
Part of the answer may be that the current generation of adaptive optics telescopes only do adaptive optics in infrared, at least if KBOs are best observed in visible light which I don't know but I've moved that to a separate question.

Regarding everyone suggesting that atmospheric absorbtion is to blame, how does that square with this: The 8.3m Subaru telescope (which was one of the telescopes used in the ground search) has a light collecting area of 53m2. The Hubble has a collecting area of 4.5m2. So atmospheric absorbtion would need to be 91.5% for them to collect the same amount of light. Sure atmospheric absorbtion is high for some infrared wavelengths, but surely not that high over all the relevant wavelengths.


texts: What object could I have seen at late June on the Sky?

texts:
In around late June, I saw two star-like bodies in the sky. At first they appeared to be twinkling, but when I looked closely they appeared to change color; blue, green, a slight red, then yellow.

It freaked me out.

But I'm wondering, did I see dying stars or was it some sort of rare phenomenon occurring because of the earth's atmosphere or was it something else?


texts: What exactly is it that is being magnified 50 times in this gravitational lensing observation?

texts:
In the Los Angeles Times news item Scientists get a rare view of a type Ia supernova magnified 50 times what exactly is magnified 50 times?

This supernova is really very far away. Is it somehow imaged by the gravitational lensing and being resolved — the actual supernova itself?

edit: I believe this is it, but it's behind a paywall (will confirm in due course): http://science.sciencemag.org/content/356/6335/291

above: From the Los Angeles Times. Photo credit: Joel Johansson

above: "This schematic image represents how light from a distant galaxy is distorted by the gravitational effects of a nearer foreground galaxy, which acts like a lens and makes the distant source appear distorted, but brighter, forming characteristic rings of light, known as Einstein rings. An analysis of the distortion has revealed that some of the distant star-forming galaxies are as bright as 40 trillion Suns, and have been magnified by the gravitational lens by up to 22 times. (Credit: ALMA ESO/NRAO/NAOJ, L. Calçada (ESO), Y. Hezaveh et al., edited and modified by Joel Johansson)" From the Los Angeles Times.


texts: Did the International Space-Earth Explorer detect cosmic rays other than gamma rays?

texts:
In looking at the available datasets described by NASA for the International Space-Earth Explorer, I am only see gamma ray burst data.

Was the platform capable of recording cosmic rays other than gamma rays? If so, where is the data?


texts: Why are the stars in my image so green (false color image, HSO palette)?

texts:
This is my first try in false color imaging. The target was IC5070 - the pelican nebula. I took around 3 hours of data (around 1h for Ha, SII and OIII). Then after stacking did an LRGB combination in PixInsight using Ha for red, SII for green and OIII for blue. For luminance I used the stacked Ha image.
Now my question is, why are most of these stars so green? It would suggest that they were really bright in the silicon spectrum right? If so wouldn't that mean that those stars are extremly silicon heavy?
(Ignore the vertical blue-green line, its a stacking artifact resulting from a camera defect on those pixels)


texts: Exposure details for this now (in)famous image from Lowell Observatory?

texts:
IAU's announcement 19035 IAU Statement on Satellite Constellations, 03 June 2019 links to 19035a Trails made by Starlink satellites which contains the image shown below.
Question: Is it possible to track down the exact start time and exposure duration for this image, and which telescope was used?

An image of the NGC 5353/4 galaxy group made with a telescope at Lowell Observatory in Arizona, USA on the night of Saturday 25 May 2019. The diagonal lines running across the image are trails of reflected light left by more than 25 of the 60 recently launched Starlink satellites as they passed through the telescope’s field of view.
Although this image serves as an illustration of the impact of reflections from satellite constellations, please note that the density of these satellites is significantly higher in the days after launch (as seen here) and also that the satellites will diminish in brightness as they reach their final orbital altitude.
Credit: Victoria Girgis/Lowell Observatory


texts: How to write G2V correctly (I mean space)? I saw in articles $ G\,2\,V$ and  $G2\,V$, on the internet webside it is usually $ G2V$.


texts: How do you measure something that has already left the observable universe?

texts:
They say that the observable universe is only 5% of the entire equation. The rest of it consists of dark matter and energy. I believe that they are trying to find a way to see something that cannot be seen due to being outside of the observable universe, sort of like how galaxies redshift and slowly face away of out existence.

tl;dr  does dark matter/energy increase every year?

What comes after red shift? Black? If so, then does that mean the universe is mostly black because 95% of matter has "Blackshifted"?

Did I post a question correctly?


texts: How would vehicles travel through the interstellar medium with its such low density?

texts:
I've heard from a number of different places that the density in the interstellar medium can have an average of 1 atom per centimeter cubed. Perhaps I have a wrong understanding of what 'nothing' is (I am very knew to astronomy/astrophysics), but how would a vehicle, like the Voyager for example, be able to travel through it?

My (perhaps very flawed) thinking behind this is that if there are so few atoms not in close proximity, there couldn't be any (meaningful) forces interacting between them to provide any sort of way to travel through it.


texts:
From the Wikipedia page about LUVOIR design:

  To enable the extreme wavefront stability needed for coronagraphic observations of Earth-like exoplanets, the LUVOIR design incorporates three principles. [...] . Third, the telescope is actively heated to a precise 270 K (26 °F) to control thermal disturbances.

I always thought the chiller a telescope mirror, the better. Why is LUVOIR heated then?


texts: How large are the effects of nongravitational forces on comet Halley?

texts:
The effects of outgassing on comet orbits can be noticeable. Yeomans, in  Cometary orbit determination and nongravitational forces, 2004, wrote:

  As the astrometric datasets improve and lengthen and as the modeling of the cometary nongravitational effects becomes more realistic, there remains the strong possibility that some physical characteristics of comets will soon be accurately determined from the orbit-determination process alone. We are already beginning to see signs that this is the case.

How much does the orbital period of comet Halley vary due to these nongravitational forces? Hours? Days?

Which comets have the largest perturbations due to these forces?

Note: a more sophisticated question along these lines is Did Rosetta improve on models of non-gravitational effects on comet 67P's orbit? - Space Exploration Stack Exchange


texts: Where do the heavier elements come from?

texts: Partly inspired by this question, the end result of stellar fusion is Iron-56 (eventually, after some decay). Then it explodes. How/where then to the heavier elements come from. The answer here mentioned two colliding stars form gold. What process is taking place to form this gold (and I'd assume the rest of the heavier elements)?


texts: What do the months and hours mean in Tomas Filsinger's Map of The Universe

texts:
I have had one of these in my room for decades:

I don't have the booklet that seems to usually come with it, can someone explain to me what the months (with days below it) and hours mean? I imagine it has to do with the position of the earth and therefore the part of the universe that is possibly observable, but I get confused when I try to get the months in the equation.

Thank you very much


texts: Why doesn't the solar azimuth vary uniformly

texts:
For the past few dates, I've been observing the rotation of the earth around the sun and noticed that the azimuth of the sun doesn't vary uniformly throughout the day. The azimuth varies faster during some parts of the day, depending on the position
link.

Why does this happen? I've been thinking about the cause, but can't come up with a proper explanation.


texts: What would be the required conditions for a lunar eclipse to occur only once on a precise annual basis?

texts:
As I am aware, lunar eclipses can take place several times a year on Earth. However, I am curious whether it would be possible for this event to only occur once annually on a specific date (and preferably on the same face of the planet) if the moon's angular orbit around Earth were to be altered from 5°.

To put it bluntly:
How would the components causing a lunar eclipse need to be altered for a singular 'blood moon' to be able to appear reliably every year on say the 25th of September? If it is at all possible.
Additionally, would these alterations cause any other significant effects between the Earth and Moon?


texts: What star is directly north of the suns north pole?

texts: If Polaris is Earth's North Star as it is directly pointed at by our North pole, what would be the North Star of the Sun?


texts: How do astronomers determine the texture of an exoplanet?

texts: So I was wondering how astronomers are able to get the texture/detail of an exoplanet or how they know a planet has water or not.  I know how they discover planets by using the light of a star and seeing if it gets faint or by seeing if the star is getting gravitationaly pulled back and forth,  to determine the planet size. But I just don't understand how they are able to get the texture of a planet. Thanks for the help!


texts: Does the observed period of a pulsar change with the time of year?

texts:
Over in the Physics SE a question was posted asking about the difference in the time dilation of the Earth between perihelion and aphelion:

Does Earth experience any significant, measurable time dilation at perihelion?

Rather to my surprise it turns out that because of the changes in the Earth-Sun distance and the Earth's orbital velocity there is a difference of about $60\mu$s per day between the two extremes.

A commentator pointed out that pulsars can be measured accurately enough to detect this difference. However I have never heard of a pulsar measurement having to be corrected for the time of year, and Googling has found me nothing related. I would be interested to know if this is something that needs to be considered.

The difference is slightly over one part in $10^9$, so presumably it depends on whether pulsars can be timed this accurately.


texts: Change in angular diameter of Cepheid variables

texts:
I'm a highschool student and in the process of writing a research paper of deriving period-lumniosity relation of Cepheid variable independently of the pre-existing PL relation. Thus, I need to derive distance to each Cepheid variable independently by using, for example, trigonometric parallax and Baade-Wesselink method.

In the process of using Baade-Wesselink method, I need to calculate the change in angular diameter, which is represented by ∆θ. The question is, how can I derive that value given fluxes from different bands (V,B, etc.. I'm using the SIMBAD database currently).

Which definite equation do I need to use to derive it?


texts: How do we know that the Universe is still expanding now?

texts:
Yes, I know that most galaxies have a red shift and that means they are moving away from us.

The problem is that the farthest galaxies are 13.8 billion light years away. That means that the info is 13.8 billion years old.

What has been puzzling me for years is that the red shift observation shows that the galaxies were receding 13.8 billion years ago. We don't have direct data on what they are doing now. It is natural that the universe was expanding with a tremendous speed at that time because it was near to the big bang, but how do we know that it still is?

If the furthest galaxies are moving with higher speed than the nearer, that means to me that their speed is reducing with the time.


texts: Why isn't there a large gap in the number of captured moons Jupiter and Saturn have?

texts:
Given that Jupiter is roughly 3x as massive compared to Saturn and is located right next to the asteroid belt, it feels that Jupiter should easily have a larger amount of captured moons. However currently, due to a recent survey of Saturn discovering 20 new moons, Saturn has 57 observed captured moons compared to Jupiters 71.

Also summing the mass of the 6 largest captured moons for each planet gives:
- Jupiter: 7.844*10^18kg
- Saturn: 8.372*10^18kg

Why isn't there a larger gap between the number and mass of Jupiter and Saturns captured moons?

Note:
While Jupiter has a very large amount of Trojan asteroids, the main focus of the question is moons directly in orbit around the main planet.


texts: Is the North star going to stay in the North sky?

texts:
Considering how the our solar system, and how every other star in the galaxy is traveling through the cosmos, it seems like it's a miracle that the stars and constellations in the sky stay as constant as they are.

How is it that the North Star seems to be fixed in space? Is it following our star through space? are we following it? Is it actually moving? Are we getting further away from it? Closer to it? What is our relationship to Polaris in the Galaxy?


texts: How long have the TRAPPIST-1 planets been in a habitable zone?

texts:
The planets orbiting TRAPPIST-1 are in orbits around their star much closer than Earth orbits Sol. However, since TRAPPIST-1 is a cool brown dwarf, some of the planets are in an "inhabitable zone", where the temperature would not be too extreme for life as we know it.

What I wonder is, how long have those planets been in an inhabitable zone? The wikipedia page says that TRAPPIST-1 is older than 1 Gyr. But, how much of that time has it been a cool brown dwarf?

In other words, since the abiogenesis of life and the evolution of living organisms has dependency on time, how much time has life had a chance to develop in the star's habitable zone?

For this question, I'm not concerned about the chemistry or other characteristics of the planets themselves.


texts: Does center of gravity of Solar system influence intensity of space rays reaching earth, space dust, speed of earth rotation?

texts:
I found an interesting article, claiming very interesting things. But I cannot find any trust-worty sources confirming such bold claims. Maybe someone came across some solid proof?

1) Center of gravity of Solar system periodically changes from almost the center of the sun to far towards sun surface (depending on planet movement)

Oscillation of the position of Center of gravity influences the following:

2) The closer Center of gravity to sun center, the more the intensity of Space rays reaching earth

2.1) Intensity of Space rays reaching earth increases water condensation in earth atmosphere (thus we have more clouds), so
2.2) Higher Intensity of Space rays makes lower temperature in near-earth-surface atmosphere (as we have more clouds), lower Intensity of Space rays makes climate warmer (less clouds).

3) The closer Center of gravity to the sun center, the more space dust (and small meteorites) gets into our atmosphere which decreases its opacity

4) Position of Center of gravity also influences speed of earth (24-hour) rotation (thru atmospheric effects as well).

Source (Russian)


texts: Could there be such thing as anti-dark matter? If there is anti regular matter, why not anti-dark matter? If there is no such thing as anti-dark matter, why?


texts: What measurements helped us in understanding 'Oumuamua's location and speed?

texts: What measurements helped scientists understand 'Oumuamua's location and speed? I wanted to know how it was achieved and with what accuracy if possible.


texts:
My question is based in this Wikipedia article about the Jupiter magnetic field:
https://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter

Jupiter magnetic field generates plasma and accelerates ionized molecules, eventually producing collisions. Is anti matter generated naturally under this scenario?


texts: How big a dish do I need for radio astronomy?

texts:
I've recently become interested in the idea of building my own small-scale radio telescope. A quick online search finds a few instructions on how to build this using a satellite dish. These suggest a dish of radius of around 1 metre, but the only objects that you seem to be able to detect are the Sun, the Earth, the Moon (possibly) and communications satellites.

This sounds like it would be become boring fairly quickly, and I would like to observe some deep-sky objects. Some example objects that I fancy observing (depending on how strong their signals are) are Betelgeuse/Sirius, the Crab Nebula and the Andromeda Galaxy.

N.B. I am quite satisfied with pointing the dish at an object and the signal strength increasing. I don't expect to get any kind of image.

Evidently, you cannot pick these up using a metre-wide dish, so what is the minimum size required to pick up these objects (noting that I can't exactly build Arecibo in my back garden)?

In case it matters, I live in a rural area, ~5 miles from the nearest town.

I originally emailed Jodrell Bank this question but they ignored me :P


texts: What is the radius of observable universe- 46 billion LY or 200 yottameters?

texts: I have found two answers for the radius of the observable universe.  Wikipedia (and other places, including this site) say approximately 46 billion LY.  I am researching this to study for the National Science Bowl, and their question database says 200 yottameters, which is about 23 billion LY (half the size that wikipedia says). There are other places which cite 200 yottameters as the radius.  Which is the more accurate answer?


texts: When will the number of stars be a maximum?

texts:
There are very roughly a "mole" of stars in the universe. Wikipedia quotes an estimate of $3 \times 10^{23}$ though the number is associated with some debate and uncertainly.

I'd like to know if there are estimates of when the number of stars in the universe will maximize. Is it expected to increase asymptotically to some maximum, or will it peak and then decrease.

I suppose this could depend on what the definition of "star" is taken to be, if brown or black dwarf objects are counted or not. I don't want to pre-specify, it's more likely that a good, well-informed answer will include this information.


texts: Calculating the radial tidal amplitude on a planet from the fluid Love numbers

texts:
How do you calculate tidal amplitudes from the fluid Love numbers? In my course on planetary physics I saw an approximate expression for the displacement from equilibrium tide:
$$\Big|\frac{V_T}{g}\Big| \approx \frac{3}{4}\frac{M_\odot R_p^4}{M_p a^3}$$
where $V_T$ is the tide generating potential, $g$ is the gravity, $M_p$ is the mass of the planet, $R_p$ the radius and $a$ is the semi-major axis of the orbit. The displacement would then be $\xi = h\frac{V_T}{g}$. This does not make a distinction between radial amplitudes at the equator or at the poles. I'm asking because I was reading this article. I was hoping the radial amplitude would correspond to my formula above (given to me without reference or derivation), but with a Love number h = 0.77 I get $\xi = 0.64$m instead of $\xi = 1.93$m.
The tidal potential in my notes is
$$V_T(r) = -\frac{GM}{a}\Big(\frac{r}{a}\Big)^2(1 + 3e\cos(M))\Big[-\frac{1}{2}P_2^0(\cos\theta) + \frac{1}{4}P_2^2(\cos\theta)(\cos(2\lambda) + 4e\sin(M)\sin(2\lambda)\Big] $$
$e$ is the eccentricity, the $P$ are Legendre polynomials: $P_2^0 = \frac{1}{2}(3\cos^2\theta - 1)$ and $P_2^2 = 3\sin^3\theta$. $\lambda$ is the longitude measured with respect to the long axis of the planet. I played around with this formula a bit, but I never find an amplitude of 1.93. It could be that this is just not what is meant by radial "amplitude".
The fact that no method of calculation is mentioned in the article makes me think it's either really obvious or really well known.


texts:
This informative answer to What was the absolute limit to the possible sizes of the first stars formed from “primordial material with no metals”? led me to Wikipedia's Quasi-star; Formation and properties which says:

A quasi-star would result from the core of a large protostar collapsing into a black hole, where the outer layers of the protostar are massive enough to absorb the resulting burst of energy without being blown away or falling into the black hole, as occurs with modern supernovae. [...] Formation of quasi-stars could only happen early in the development of the Universe, before hydrogen and helium were contaminated by heavier elements; thus, they may have been very massive Population III stars.

But neither the answer nor Wikipedia plainly state just what it is about the presence of metals that would make Quasi-stars impossible, such that if they ever did exist it would only have been in the early universe.
Question: Why could Quasi-stars ("black hole stars") have only existed when everything was hydrogen and helium? (no metal "contamination")


texts:
In the far future the Milky Way and Andromeda will collide.
As far I know direct collision between large dense objects (black holes, stars, planets, etc.) is unlikely due to the relative "low density" of both galaxies.
My naive intuition is it's like two big clouds of dust merging.

I know at the collision time the Sun will be most like a red star and Earth a scorched dead planet.
For the sake of the next questions let's hypothesise another system with a star resembling the Sun and another small beautiful third rocky blue planet holding some advanced live form.
Also, for acceleration measures, let's use a frame of reference like if someone is observing from far away both galaxies getting close one to another and interacting.

Some questions pop up in my mind.

Can the gravitational interaction sling shot an entire stellar system? How much Km/s2 we can expect these system to accelerate? (In a most extreme plausible case).
How this acceleration can affect the system? Can it de-stabilise planet orbits for example?
How it can affect life? Can it affect magnetosphere? change it's shape? Can solar flares/wind get stronger in some path due particles acceleration?
If another star passes really near (let's say 1 light year at closest approach) but at great speed can its light reach that planet at high energy levels, let's say x-ray and affect that planet life?


texts: If Planet Nine is a primordial black hole could it explain 'Oumuamua and Borisov's orbits without them being interstellar?

texts: There has been an increasing amount of interest in the possibility of primordial black holes and that Planet Nine may be one itself.  Planet Nine is a potential explanation for the orbital characteristics of various TNOs (trans-Neptunian object).  I'm curious if it would be possible for our two interstellar visitors ('Oumuamua and Borisov) to be from this solar system but have been given a gravitational assist by a small black hole to provide them with their high eccentricity.  I further wonder if the elongation of the low density 'Oumuamua could be due the effects of a close orbit around a small object of unusual large density.


texts: During an eclipse, how big is the shadow of the moon on the earth?

texts:
This picture was taken from the ISS during a solar eclipse.

You can see the shadow of the Moon on the surface of the Earth. But how big is this shadow? How many kilometers is its diameter?


texts:
Both Quito and Kampala lie on the Equator. The longitude of Quito is 82°30'W and that of Kampala is 37°30'E. What is the distance from Quito to Kampala?
A) along the shortest surface path
B) along a direct through the Earth path?
Please help me visualize it with a figure. How to do it?


texts: Are black holes expected to contain the same ratio of dark matter to regular matter as the rest of the universe? I've heard that dark matter is distributed in halos around galaxies. Does that make it less likely to be ingested into a black hole?


texts: Question about total absolute magnitudes of galaxies - negative or not?

texts:
I have seen this in a couple of places, but when the authors in the paper - http://chandra.as.utexas.edu/~kormendy/kfcb-accepted.pdf -
wrote:

  "All 10 of our ellipticals with total absolute magnitudes M_VT ≤ −21.66 have cuspy cores – “missing light” – at small radii. Cores are well known and naturally scoured by binary black holes formed in dissipationless (“dry”) mergers. All 17 ellipticals with −21.54 ≤ M_VT ≤ −15.53 do not have cores."

Since absolute magnitude is the magnitude of that object at 10 pc away from Earth, then this is why the galaxies have such negative (meaning, very bright) values for total absolute magnitude, correct?

However, in this paper - http://arxiv.org/pdf/astro-ph/9602044v1.pdf - the author writes:

  "The morphological properties of galaxies between 21 mag < I < 25 mag in the Hubble Deep Field are investigated using a quantitative classification system based on mea- surements of the central concentration and asymmetry of galaxian light."

The galaxies here are between 21 to 25 absolute magnitudes (?) in the infrared (I?) band, but why are these magnitudes not negative?


texts: How to understand exactly why gravity darkening happens on rotating stars?

texts:
Phys.org's TESS delivers new insights into an ultrahot world links to KELT-9 b's Asymmetric TESS Transit Caused by Rapid Stellar Rotation and Spin–Orbit Misalignment (readable in arXiv)
The assymetric dip in the light curve comes from a near-polar transit across a rotating, oblate star where the poles are hotter and therefore brighter due to gravity darkening:

KELT-9’s high internal angular momentum ($\nu \sin(i)$ = 111.4 ± 1.3 km/s) flattens it into an oblate spheroid, making the equatorial radius of the star larger than the polar radius. Additionally, the star’s abundant centrifugal force near its equator distorts its hydrostatic equilibrium, causing its effective temperature to vary by nearly a thousand Kelvin over the surface of the star. These two effects of stellar oblateness and varying effective temperature – together commonly referred to as gravity darkening (Barnes 2009) – change the total irradiance on KELT-9 b (Ahlers 2016).

Those links discuss gravity darkening but don't offer a simple explanation.
Wikipedia's Gravity darkening says:

When a star is oblate, it has a larger radius at its equator than it does at its poles. As a result, the poles have a higher surface gravity, and thus higher temperature and brightness.

Question: Why exactly does the increased surface gravity at some places on a given star lead to a higher temperature at those locations? Is it related to the difference in scale heights? The surface brightness relates to the temperature at the photosphere, is the reason simply that a higher pressure therefore higher temperature is needed to support the same density in a higher gravitational field?

Figure 2. (Left) KELT-9 b begins its transit near the star’s hot pole and moves toward the star’s cooler equator. Our transit analysis directly measures the stellar inclination (i★), the planet’s projected alignment (λ), and the orbital inclination (i.e., the impact parameter b). We find that KELT-9 varies in effective temperature by ∼ 800 K between its hot poles and cooler equator. (Right) KELT-9 b’s phase-folded primary transit from TESS. The transit depth steadily decreases throughout the eclipse, indicating that KELT-9 b begins its transit near one of the host star’s hotter poles and moves toward the dimmer stellar equator.


texts: How big are the tidal influences of planets (say, Jupiter) on the sun? Do they lead to any measurable effects, for example, do they influence the sunspots, or are they observable in any other way?


texts:
This answer to the question Is Io a magic energy machine? suggests that the energy from the internal heating of Io due to tidal "squishing" as it moves cyclically closer and farther from Jupiter in its elliptical orbit will come from the energy of Io's orbit. A lower energy orbit is necessarily smaller, and that actually means that the velocity will be larger. (When you want to raise a satellite's orbit to a higher altitude, you actually use thrust in the direction of motion to slow it down.)

Given that tidal forces are a little complicated (cf. Why is the Moon receding from the Earth due to tides? Is this typical for other moons?), is it a priori certain that the heating will lower Io's orbit, causing it to speed up? (Consider that Earth's Moon's receding is due in part to Earth's liquid ocean, and Jupiter is a gas giant.) Is it just the perijove that will decrease, or the semi-major axis?

How can a (seemingly, naively, on average) radial force cause a tangential acceleration? Io is tidally locked to Jupiter so its rotation around its own axis is synchronous to its rotation around Jupiter.

edit: fwiw if the gravitational interactions between Io and Jupiter's other moons causes the problem to be too complex to answer easily, I'm more interested in the basic dynamics of tidal heating and effects on the orbit of a single moon, rather than specifically Io's situation.


texts: Can anyone recommend me books or other online resources on the concept of the Multiverse, it types, and about higher mathematical dimensions of space-time. I have been interested in such topics for a while but have not found any accessible book that could explain them in a way that's comprehensible to me. (I'm new to the subject of astrophysics, just getting started).


texts: Force exerted by space debris on satellites

texts:
Hypothetically, if space debris were to collide with the currently orbiting satellites, how much force would they exert, and how much damage would they make?
Will the satellites be able to withstand that much force, especially if they don't have Whipple shields?
Since space debris travel at tremendous velocities, I am guessing the force is pretty large too. How do you calculate the force? Is it a simple $\rm{F=m \cdot a}$ calculation?


texts: Did the night sky ever change in recorded history?

texts: I wonder whether there has ever been a major change of the firmament in recorded history, like changes in the positions of stars, changes in constellations, or stars disappearing after going supernova. There've been visible supernovae but had the star in question been visible to the naked eye before the supernova? If so, how did it change what constellation?


texts: Is it just coincidence that the Moon follows the ecliptic?

texts: Or has it been gravitationally steered that way because of how it was formed?


texts: Is there a place in the known universe that there would be no visible light from other stars/galaxies?

texts:
Is there a place in the universe where, with the naked eye, you wouldn't see any stars, galaxies or light emitting phenomena? From what I understand at some point into the far future in Earth's location, we wouldn't be able to see anything. Is there a place like that could be like that now?

p.s. Not sure what the appropriate tags to put on this question. Feel free to add/change tags as needed.


texts: Qualities of gas giant atmospheres at a density of 1 gram per cubic centimeter

texts:
At what depth in the atmospheres of the gas giants does the density of their atmospheres equal 1 gram per cubic centimeter? What is the pressure and temperature at these depths?  Are the radiations levels congenial to human life at these depths?
Wikipedia's Saturn; Physical characteristics says

Finally, the lower layers [of the Saturnian atmosphere], where pressures are between 10–20 bar and temperatures are 270–330 K, contains a region of water droplets with ammonia in aqueous solution.

What is the density of the atmosphere in the lower atmosphere?  The lower atmosphere of Neptune is 275K at 50 bar.


texts: Is it coincidence that Einstein predicted exactly twice the outcome of Newton for the gravitational deflection of light?

texts: During a solar eclipse Sir Arthur Eddington measured the deflection of star light by the Sun. Einstein predicted a deflection of 1.74 arc seconds, but according to Newton the deflection should have been 0.86 arc seconds. So is it a coincidence that the prediction of Einstein is twice as large or is there a kind of law involved which makes Einstein's outcome of his GR twice as big as that according to Newton?


texts: What is the largest hydrogen-burning star?

texts:
I am wondering what is the largest known core hydrogen-burning star?  A look at the list of largest known stars on Wikipedia seems to indicate VV Cephei B (at the bottom of the list), but I would like to know for sure if it is the largest known.  In addition to knowing which star it is, I would also like to know its temperature, size, and expected lifetime.

I am also curious to know if the largest known core hydrogen-burning star is similar to what astrophysicists theorize is the largest possible core hydrogen-burning star (given current metallicity conditions in the universe; I know star-formation timescales have a metallicity dependence) and the expected temperature, size, and lifetime of such an object.


texts: Where can I find galaxy spectra (in the form of FITS files)?

texts:
I know there's SDSS and I tried searching for the spectra through the navigate and explore tool under CAS as well as by using RA/Dec coordinates under SAS, but every single time it comes out with 0 results, not part of the database or out of the SDSS threshold.
Moreover, I do not know where I could cross-reference to get the SDSS object id based on other common identifiers (such as NGC) in order to refine my search since I have been solely using RA/Dec coord. as of now.
Any kind of help on how to look for (interactive) spectra would be greatly appreciated. I wanted to use this to calculate the recessional velocities (and rotational velocities by observing H-alpha emission) of different galaxies.


texts: Theoretically, what is the biggest optical telescope that may exist?

texts:
Recently, I read yet another news about E-ELT. It will have 39.3-metre-diameter segmented primary mirror. And I was interested in the next question:
Theoretically, what size of the primary mirror (single/multiple/segmented) can have a telescope on Earth for observing at optical wavelengths? And why? I mean what are physical limitations exists?

And the same question about space (not on Earth)?

Update:

On the advice of @TildalWave, to make this question answerable let's make a few adjustments:

Primary mirror should be segmented (or it variations) like on E-ELT.
Suppose we have a large (several square kilometers), flat surface high above sea level.
We have to build telescope for observing at optical wavelengths.

I know that, there is concept of OWL with 100 metre-diameter segmented primary mirror.

But what about 500 metre-diameter or 1000? Is it possible in theory?


texts: GW190521's Luminosity Distance is greater than the age of the universe - how?

texts:
According to this article about the gravitational wave observation, GW190521, the estimated luminosity distance is appx 5+/-2 Gigaparsecs, which is appx 16 billion light years +/- 6:
https://dcc.ligo.org/public/0165/P2000020/012/LS17910.pdf
While I understand that this is a crude measurement, is it not true that the distance cannot be greater than the appx 13.7 billion light years of the universe?  If so, should not there be another constraint on the distance? Or is it just more accurate to report the calculated luminosity distance and leave it at that in the report?


texts: Is the ratio between earth's distance from the sun and the speed of light just a coincidence?

texts: I was doing some calculations to see how hard it would be to observe the speed of light and discovered an interesting correlation between the speed of light and the average distance from the earth to the sun. In my example, an object would be placed in orbit around the sun so that its  shadow would pass over the earth at the speed of light.  After doing some math, I found that an object roughly 1 km above the sun, orbiting at 2 m/s would achieve this.  I though that these numbers were a bit odd, so I took a closer look at the speed of light (299,792,458 m/s) and AU (149,597,871 km.)  I found that 299,792,458 / 149,597,871 = 2 within a .2% degree of inaccuracy.  To me, this seems too close to be chance.  Is there an established relationship here, or is this just luck?


texts: How do I calculate the positions of objects in orbit?

texts: In a model of the solar system, given the Sun is at the point $(0, 0, 0)$, an given the six orbital elements for each object in orbit ($a$, $\epsilon$, $i$, $\Omega$, $\omega$, $M_0$), how can I calculate the relative position to the sun at a given time $t$?


texts: What exactly is the cosmic definition of ‘now’ is?

texts: There is no cosmic definition of now because of special relativity, because of rate at which time flows for a particular object depends upon your point of view and your velocity. For a rocket traveling at speed of light, journey of alpha centauri might take a week. For us it would seem like 4 years. As a result of special relativity even the meaning of ‘simultaneous’ is related to your point of view


texts: I was looking to Scorpio the other day, and noticed that the last star in the tail (the telson's tip) -- known as Upsilon Scorpii, υ Scorpii, or Lesath -- seemed yet more red than the "heart" of the scorpion -- Antares or Alpha Scorpii. Was it that I was drunk, or some kind of sky pollution? (Actually, a friend confirmed it was red, and she seemed sober than me.) Or is it, indeed, red? I couldn't find any information on-line. I saw this database, but don't know how to find a star by its name, or if they have information about the color.


texts: Could something temporarily break the Moon's tidal lock?

texts:
Would it be possible for something to cause the Moon to start rotating at a different rate, breaking its tidal lock with Earth, without ripping it apart?

If so, would the Moon's tidal lock eventually stabilize?


texts: What is the apparent rotation time for a planet relative to an other planet?

texts:
Just my curiosity. This is not my assignment etc.

Let planet A take m Earth days to complete one full round around the Sun, and planet B take n Earth days for one full round around the Sun. Assuming that both are orbiting in the same direction, what is the duration recorded by an observer on the planet A, as the number of Earth days the planet B took to complete one full round around planet A?


texts: I was out taking some milky way pictures last night, and saw a much quicker shooting star type thing, only with a much more random movement - any ideas what this could be - this is the RAW file from my camera - converted to JPEG for upload


texts: Why does radio astronomy offer higher resolution images than optical?

texts:
According to this lecture,"Radio astronomy has several advantages over optical astronomy...We can make the highest resolution images, and see things happening on the smallest scales."

Why is this? If resolving power, $r \propto \frac{\lambda}{B}$ then shouldn't shorter wavelength light result in smaller angular resolution i.e. better resolution?


texts: If one galaxy were to collide with another, what would it look like to an immortal inhabitant of one of their planets?

texts:
I was reading this article posted on /r/space about photographing black holes, and they showed a picture of what's believed to be two black holes formed from two galaxies colliding.

I didn't realize the collision of galaxies could result in black holes, but it got me wondering. If you lived in one of those galaxies, what would happen to you? Would your planet be destroyed in the blink of an eye at some point? If so, when?

Would the sky gradually fill with more and more stars? If your galaxy became a black hole, how would you experience that? All at once or very gradually?


texts: Do unattached objects contribute to a planet's gravity?

texts: If you could measure Earth's gravity with infinite precision, and did so both with Earth and its full population, and Earth with no people, would there be a direct difference in gravity? Is this even a valid way to think of gravity?


texts: How could we search for life in TRAPPIST-1

texts:
It is possible to search for life from here, how?
What methods could be used?
It is possible to look for organic matter?
Could we know if there is life according to the components of the planets atmosphere?


texts: How would a planetary defence against comets work?

texts:
Mappings of practically all potentially dangerous asteroids is well underway. But comets seem almost hopeless to protect against.

How far away are approaching comets detected today? How long time before they would pass Earth orbit?
Is it feasible in our time to map comets in their beyond Neptune orbits, or will we only detect them while approaching?
How much more difficult is it to reach a comet compared to an asteroid? I imagine that the high speed and potentially high inclination of a comet is a challenge for rocketry. Rosetta reaches a comet soon after 10 year trip, but I suppose they picked the easiest to reach comet known.


texts: If Earth had rings, how long would it take for the rings to loose orbit? One of the answers in this question https://physics.stackexchange.com/questions/281828/if-earth-had-rings said we did at one time is it possible?


texts:
If I'm on a life boat, in the middle of the ocean, radioing another ship to come pick me up - I'd say something like "I'm 40 kilometers directly east of you."

In space, it's not that easy. How do you give a direction?


texts:
Last night we saw a large cluster of what looked like exceptionally bright stars that moved, as a group, across the night sky in Lancaster, PA. It was about 9:15 pm, and they moved roughly from SW to NE. There were 25 or 30 of them. They moved as quickly as a satellite I once saw, but were bigger and brighter. They were soundless and did not blink, and the relationships between the lights changed slightly as they moved.

We don't believe in UFOs, but can't we find any explanation via google.

Can that many satellites move as a group?


texts: I've learned that the time period for a rotating neutron star, i.e., a pulsar is very accurate and can be used as an interstellar clock. But according to Walter Lewin's 8.01 Lec 19 on classical mechanics, he says that the rotational time period of neutron star in the Crab Nebula loses up to 364 ns everyday in the form of neutron star emissions (this comes from rotational kinetic energy of the neutron star). So, how are pulsars used as precise clocks?


texts: Might the SpaceX Roadster intercept any ongoing surveys?

texts:
There are star survey telescopes in space, and on Earth. I don't really know how to begin to know where they will all be looking for the next few months. On the off-chance that someone does, or can suggest how to find out in a systematic way, I thought I would ask here.

These is JPL's Horizons predictions for the Roadster spacecraft in its heliocentric orbit. They are based on both the last state vector readings from the spacecraft itself plus several optical sightings my medium sized telescopes.

While the car is fairly small, it's still attached to the white, 2nd stage rocket body, so it continues to reflect a bit of light. You can read more and see some astronomical photos in this answer and read this answer for info.


texts: Catalog of Galaxies in the Visible Universe

texts: I have seen the Gaia project from here which is about mapping the stars in the Milky Way galaxy. But I'm wondering if there is any open data on the positions (or something like "positions" since it probably not calculable now), or "coordinates", of the galaxies in the visible universe, as opposed to the stars. I wonder how animations such as this (a time=1m30s point it shows the galaxies zooming out from the Milky Way) get their data on the positions of the galaxies.


texts: Why is the moon phase not in sync with the moon's illuminated fraction?

texts:
I am using the following R script to plot the moon phase and the moon's illuminated fraction:

library(oce)

X11()

t_start <- as.POSIXct("2017-02-23", tz='UTC')
t_end   <- t_start + 75 * 24 * 3600  # 35 days

tm <- seq(from = t_start,
           to  = t_end,
           by  = 10000
        )

moon     <- moonAngle(t=tm, longitude=-8, latitude=47)
fraction <- moon$illuminatedFraction
phase    <- moon$phase - floor(moon$phase)

plot (tm, fraction, type="l", col='blue')
lines(tm, phase   , type="l", col='red' )

grid()

# wait for mouse click or enter pressed
locator(1)

This is plotted:

The red lines indicate the phase (0 = new moon, 1/4 = first quarter, 1/2 = full moon etc.), the blue curve indicates the how much of the moon is illuminated.

I have expected the illuminated fraction to be zero exactly then when also the phase is zero. However, this is not the case.

So, is there something I am missing, am I doing something wrong in my script or is the implementation of oce inaccurate?


texts: Is there a Free repository of pictures taken everyday of the night sky?

texts: I'd like to know if there's a repository out there of pictures taken everyday of the night sky. Not looking forward to anything fancy. Maybe a group of people out there have set up something out of their home telescopes or something.


texts: What are the requirements to become a planet?

texts: A planet is taught to us (at primary school) as a ball of rock that orbits around the Sun and is similar to our own planet, Earth (in as a ball of rock). But of course, humans have to be more specific than that so there must be some internationally agreed on requirements to define a planet by an official committee, right? I know of an example of this is when in 2006, these requirements were enforced and Pluto got demoted from planet status to dwarf planet status.


texts: Does anyone knows the origin of the Maunder Minimum and Dalton minimum?

texts: Solar spots seams to be a good indicator of solar activity. Does anyone know the reason for the Maunder and Dalton minima?


texts: The mass of Subdwarf B progenitors in the main sequence

texts:
Note: By "progenitor", I mean "the star as it was when it was still main sequence". Please correct me if I've got the terminology wrong.

Most B-type subdwarfs have mass roughly half that of the Sun. Prior to becoming blue-white subdwarfs, they went through the red giant phase, so their main sequence masses would have been between $0.3$ and $8M_{\odot}$. Since they went on to fuse helium, we can raise that lower bound to $0.5 M_{\odot}$. Since helium fusion in these stars begins with a "helium flash", then they would be $< 2M_{\odot}$ during the main sequence.

Is anything more known about the masses of main-sequence stars that can become B-type subdwarfs? The narrow range of masses observed for the subdwarfs derives from the narrow range of viable core masses. However, the papers I have read indicate that the progenitor stars may have masses in a much wider range than that of their cores. So the $0.5M_{\odot} \leq M \leq 2M_{\odot}$ range given above may be the tightest-known bounds for the mass of such a star, but if anyone is able to provide a reliable source for tighter bounds, I would be very interested.

Source for the $0.5M_{\odot}$ lower bound:

Laughlin, G., Bodenheimer, P., & Adams, F. C. (1997). The end of the main sequence. The Astrophysical Journal, 482(1), 420.

Source for the $2M_{\odot}$ upper bound:

Harpaz, A. (1993). Stellar evolution. AK Peters/CRC Press.


texts:
Can someone help me? I have a case, like, we have H-R diagram and two stars on it. One of them is a blue star on main sequence, another is a red giant. They have equal luminosity. If they are in a double star system and seen from Earh as one star, then where on the diagram should I place this star? What luminosity amd temperature will it have?

Maybe I should sum up their luminosity and then, using Stefan Boltzmann law, calculate the temperature? But does the law work for red giants? And can I sum the luminosity?


texts: A new temporary moon named 2020 cd3 has been discovered. How can we determine that this is temporary and why does it have such an orbit which is such irregular in shape ?


texts: I'd like to one day take the Astronomy Olympiad, however I'm at a loss to know where to begin. As it currently stands I have no experience whatsoever with Astronomy. If there are people here with some expertise on the matter, I'd like to know what books you would recommend to study? The syllabus is at the following link: http://www.ioaa.info/page2/page7/page7.html


texts: Black holes and super massive black holes

texts: What is the difference between a black hole and a super massive black hole?


texts: How would seasons at the equator be if the Earth were tilted 90 degrees?

texts:
If Earth's axis of rotation were tilted 90 degrees, what type of seasons would be observed at the equator.

My prediction is that there would be no seasons, as the orientation of the equator would be the same as if the Earth


texts:
https://en.wikipedia.org/wiki/Universe
Is the Universe infinite or finite ?


texts: Was the discovery of six exoplanets around one star as "easy" as counting six peaks in the FT?

texts:
The phys.org article Scientists make huge dataset of nearby stars available to public describes the release of a publicly accessible database of Echelle radial velocity measurements; The LCES HIRES/KECK Precision Radial Velocity Exoplanet Survey. See also Keck's HIRES home page.

  For two decades, these scientists have pointed HIRES at more than 1,600 "neighborhood" stars, all within a relatively close 100 parsecs, or 325 light years, from Earth. The instrument has recorded almost 61,000 observations, each lasting anywhere from 30 seconds to 20 minutes, depending on how precise the measurements needed to be. With all these data compiled, any given star in the dataset can have several days', years', ore even more than a decade's worth of observations.

This part caught my interest especially:

  "We recently discovered a six-planet system orbiting a star, which is a big number," Burt says. "We don't often detect systems with more than three to four planets, but we could successfully map out all six in this system because we had over 18 years of data on the host star." (emphasis added)

For very simple cases of one, or maybe two planets with minimal interplanetary gravitational interaction, a Fourier transform of a nice, long, continuous radial velocity measurement would show two main peaks, and possibly other artifacts. If the stellar motion induced by each planet were similar magnitude, the analysis might be fairly simple.

But for the six planet case mentioned in the quote (I don't know which one it is), and patchy time coverage (it's a survey) how was this analysis done? Peaks alone? Or just throw it to a supercomputer simulation of every possible combination and let simulated annealing run for a month?

Or was there some 'detective work' involved as well - assumptions, limitations of the fitting space, or even inclusion of other data from outside the study?


texts: How distorted will our galaxy be if we are viewing it from several thousand light years away?

texts:
I saw this answer and read this sentence regarding the distortion of our galaxy when viewed above the galactic plane:

Once you got to a point where the entire [sic] galaxy was within your vision, the entire galaxy would look distorted, since our galaxy is 100,000 light years [sic] across, the light from the edges of the Galaxy would be thousands of years old, and would not match up with how the spiral should appear.

I am thinking that this could potentially be true, but I am not sure why or how distorted it would be. Does this fact hold, and for what distances will it no longer be distinguishable?
Note: I have thought that it will be well-blended because the distance does not spike, it increases gradually, but I need verification.


texts: Transform pixel coordinates (in FITS file) to equatorial

texts:
Say I have a FITS file with an observed field of stars, each one with x,y coordinates given in pixels. Assume the image is aligned so that the x axis is the right ascension, and the y axis the declination.

I'm looking for a tutorial to transform the pixel coordinates of each star to equatorial, even if the transformation is approximate.

I'v been looking for a while but found no tutorial/book/site that enumerates the steps needed to produce this transformation.


texts: Finding the temperature of Earth from temperature of Mars and its distance from Sun

texts:
I am to compute the temperature of planet--in fact I've already found out it's Earth--knowing only:

the surface temperature of Mars (210 K) and its distance from Sun (1.524 AU)
and of course distance of Earth from Sun (1 AU).

I have no data concerning the temperature of Sun, surfaces of the planet, etc. What formula should I use? Thank you.


texts: Does Dark Matter affect Space Time Curvature? If so, then what is the formula to calculate space time curvature due to dark matter?


texts: What is the Fundamental plane for Elliptical Galaxies?

texts: An alternate explanation rather than from wikipedia is preferable.


texts: Where can I find a visible star dataset?

texts: I'm trying to do a project for a databases class where I have a user enter their current location and I tell them what constellations/planets/etc are visible in their area. I've seen this data compiled into images, PDFs, and website entries, but I can't seem to find a csv for it. I get that this data is bound to be massive or extremely complex, but I feel like there should be something out there. Any clues?


texts: How many degrees does the sidereal month orbit?

texts:
It's written everywhere that anomalistic month orbits 360 degrees completely, then how many degrees does the sidereal month orbit?

Here is written that the sidereal month orbits 360 degrees too, but it doesn't make sense that they have the same orbit degrees since the anomalistic month is longer.

According to my logic, if 360 degrees equal to 27.55455 days, then 27.321661 days (=sidereal month) equal to 356.9 degrees, rather than 360 degrees. Is it correct?


texts: Use of type-I a supernovae as standard candle

texts: Why only type -Ia supernovae are used as standard candle to calculate age of universe and why not type II, type Ib, type 1c ?


texts: Can automatic algorithms completely eliminate the impact of Starlink and other satellites?

texts:
According to a comment by user @J:

With that said, other than aesthetics, machine vision algorithms are wildly more advanced today than in the past - strategies to remove passing satellites don't really seem to be anything other than trivial. It's annoying because it requires land based observatories to correct for the new noise, but it doesn't seem anything fundamentally more disruptive than that - a mild annoyance.

Is it true that machine learning (and other) algorithms are so good these days that you can automatically eliminate the impact of any satellites passing overhead? Perhaps not the ideal solution for a casual astronomer but perhaps easy to do for a professional observatory?


texts: Origins and most frequently used; perinigricon vs peribothron?

texts:
Gizmodo.com's Astronomers Spot Unprecedented Flashes From Our Galaxy's Black Hole mentions Sgr A*'s companion gas cloud G2 and that Wikipedia article uses the term perinigricon, but that mentions peribothron as a synonym.

So which term for an object's orbit's closest approach to a black hole is most frequently used; perinigricon or peribothron? And what are the origins of these competing terms?


texts:
Hey I want to describe an event A happening roughtly 5-7 after event B. I'd like to describe it as the 'A happened after the sound of B echo'd off X', where X is recognisable space thingy...

(I'm talking mildly poetically, so I'm aware there is no sound in space, but I'm interested in what X might be, if such a thing was possible - if it makes you feel better you can use light and I'll describe 'A happened before X saw B') :)


texts: Four Lights (star like) Moving in a Line Together

texts:
About 20 minutes ago, we saw four lights lined up and moving relatively quickly across the sky. These lights looked exactly like the stars around them - not red or blue or flashing or anything, just four stars at first glance. The spacing between them remained constant as they traveled in a straight line together (moving from West to East). The two in the middle were slightly closer to one another than the two in the front and back of the line, but the spacing was otherwise equal and constant - as if they'd been strung together on a line that was begging pulled forward.

They moved as quickly as a plane might when making a final approach to land, but there were no other lights visible around them and no sound. After watching for a minute, I called a friend out to reassure me I wasn't just seeing things, but they saw the same and described them as resembling stars more than anything else as well.

Do satalites travel in groups? Would they form a consistent line like this? That's the next closest thing I can think of, but I've never seen satalites quite that bright/large or traveling in locked step with others.

Could these have been military planes flying high enough to only appear as one whitish, twinkling light but still as fast as a lower flying plane? There is a naval Base not to far from us; that's the only other thing I can think of.


texts: After a supernova, why don't new stars coalesce with higher heavier element content?

texts:
May be a beginner's question but I couldn't find the answer anywhere.
Given a supernova event and the cloud left by it, why does a new star coalesce with a disproportionally higher hydrogen(and helium) content compared to the matter left on it's vicinity?
Shouldn't gravity form bodies(maybe stars still) with all, or most of, the heavier elements and leave the hydrogen and helium in the vicinity instead?
I would also appreciate any book indications on that matter.


texts: What's the furthest distance that something could travel and eventually come back to Earth?

texts:
Imagine u shoot a photon into the sky at a mirror far away in space, and you want the photon to bounce off the mirror and eventually come back to you. Considering the cosmological constant, what's the furthest that mirror could be (at the moment you shot the photon) and still have the photon get to it, bounce back and eventually return?
I know the mirror would start moving away, and the photon would have to catch up with it. I think it would have to be around 15 BLY away by the time the photon reached it (I'm not sure), but I don't know how far it would have to be at the start to be 15 BLY away by then.


texts: What are the most popular galaxies for which we have images?

texts:
The only galaxies I can think of (not being an astronomer) are Andromeda and Milky Way. There are 51 near galaxies, but they all pretty much say "satellite of Milky way" or "satellite of Andromeda". There are 100k+ galaxies in the local supercluster, and that page seems to have a better list:

Corvus
Coma Berenices
Ursa Major
Virgo
Sculptor
etc.

If you had to rank them in order of prominence in the scientific community or in popular science, wondering what the top 10 or 20 galaxies would be (for which we have photos).

I am trying to come up with a list of images for educational purposes that are potentially somewhat familiar to laymen audiences, or which would be useful to introduce to laymen audiences.


texts: Where can I download light pollution data?

texts:
I'm creating a website with prediction of how good will be a night for astrophotography.
I take into account the Moon, clouds,  but of course I need light pollution data.

Where can I download a light pollution dataset or may be use some API?


texts: What may be the period and speed of revolution of the star that is closes to galactic centre?

texts:
The Sun moves at the speed of 220km/s around the galactic center in a circular orbit. The sun is located at about 30,000 light years from the galactic center. The Sun orbits around the center of the Milky Way once every 225 million years. The period of time is called a cosmic year.

I am curious about the speed at which the star closest to galactic center revolves around galactic center. Has this been calculated or approximated. Also what is the time it takes to complete one revolution for the star around galactic center or what is the cosmic year length for the star that is closest to galactic Center .

I have taken reference from http://www.bibliotecapleyades.net/ciencia/ciencia_astrosciences07.htm


texts: Universe Sandbox 2 accurate representation of gravity

texts:
I've been playing Universe Sandbox for a while now and I have never been able to make a near Earth asteroid collide with it. By "make" I mean, simulating billions of years of known, near Earth objects until one of their orbits intersect. In this explanation Sir Cumference describes that Universe Sandbox 2 uses Newtonian physics to in its simulations. But wouldn't that calculate enough pull from Earth to slightly change the trajectory of the NEO to eventually impact?

We know, with almost 100% certainty that a LARGE NEO will impact Earth (probably sooner than later), otherwise why would we be investing so heavily on the Jet Propulsion Lab's Asteroid Watch program.

Can the game not accurately simulate gravitational pull on those large scales or is there something else (probably a lot) I'm missing? Are all known NEOs not projected to have a relevant collision intersection with Earth?


texts: Why is there a black stripe in the Hubble images of Pluto?

texts:
While reading reports about the New Horizons misson, I noticed an odd vertical, black stripe in the images of Pluto.

Here is an example:

Source: Hubble Discovers a Fifth Moon Orbiting Pluto (07.11.12). Credit: NASA; ESA; M. Showalter, SETI Institute

Why is that black stripe there?


texts: What's the cheapest lens to buy for showing ISS to a flat earther?

texts:
Recently I entered a discussion with someone that has a relatively popular YouTube channel which says among other things that the earth is flat. I said that earth isn't flat because you can see the ISS by yourself through the telescope. I was reminded that I'd only seen it on the internet, but not actually seen it directly myself. I was told that if I buy a telescope and film by myself they will also buy one, do the experiment, and post the footage on their YouTube channel.

So, what's the cheapest telescope that I can buy and record with a cellphone? I've seen people using a Nikon P9000 but what matters is its lens, which is already expensive, and the camera is expensive too. Maybe it's possible to buy a cheap photography lens that has both good quality and high zoom and can be used with a cellphone?


texts: What is this "Table of astronomy" about?

texts:
I found this picture while looking for some tattoo ideas and would like to know what these sketches are supposed to mean.

Im especially interested in figures 1, 21, 53 and 56.

It appears to be a scan of page 164 of the Cyclopædia (an Universal Dictionary of Arts and Sciences), from 1728. Higher Resolution


texts: What constellation would the Sun be in from an exoplanet?

texts: Where would it be in the sky from Proxima B (planet near Proxima Centauri) for example?


texts: Do all stars of a given age and mass have relatively the same emission spectrum?

texts:
My understanding is that much of the visible light that we see during the day is given off from the Sun, and that means that most of the colors that we see are dictated by (with the exception of atmospheric effects) the particular wavelengths of color that the Sun emits, which does not include the entire continuous color spectrum.

Here is an image of the wavelengths emitted by the Sun

My understanding is also that, at least during a particular part of the life cycle of a star, all stars are burning balls of hydrogen and helium. If that is the case, then do all stars at that point in their life give off emission spectra of visible light that is similar to our own? Is it possible that an observer on a planet with an atmosphere similar to that of Earth might see different ranges of color than we do, simply due to, perhaps, the composition of their star? Could there be planets similar to Earth where the sky would appear purple rather than blue?

Let me know if any of my assumptions are incorrect.


texts: Density of a sunspot compared to the surrounding photosphere?

texts: Is the density of a sunspot different by more than a factor of ten compared to the area surrounding the sunspot?


texts: How bright was Scholz when is passed near the Sun 70,000 years ago?

texts: I just read that a star called Scholz passed only 0.8 years-light away from us 70,000 years ago.  It is a red dwarf. I don´t know how bright it was in the night sky. Does someone know how bright it was?


texts:
Some meteorites just blaze away, streaking through the sky. This is clearly explained, as they either slow down enough for the flame to cease, or ablate in their entirety, turning to vapor.

But some, like the Chelyabinsk meteorite, explode violently.

What effect is responsible for this?

Can it be that with their irregular surface acting as a turbine they enter a spin of increasing speed, and once the centrifugal force reaches a certain level, it causes the meteorite to break apart rapidly, hurling parts of it in all directions at speeds far exceeding its speed so far?


texts: Adaptive Optics observations worse than natural seeing observations for faint structures?

texts:
I have read that one drawback of Adaptive Optics (AO) IFU observations is that they have lower SNR (compared natural seeing observations) and as a result it is harder to observe faint structures. For example, in a galaxy observation, while AO can give great resolution in the inner parts of the galaxy, the faint outskirts or the disk are more visible with natural seeing observations. Is that true? Why AO are not good with faint structures? I assume there is some loss of light during the whole process involved with AO which results in lower SNR? Can someone give me more details please?

EDIT1:

In this paper: http://adsabs.harvard.edu/abs/2011A%26A...528A..88G The authors say:

  In the galaxies from the LSD sample we find more complex morphologies
  which are likely the consequence of the AO-assisted observations
  providing higher spatial resolution (∼0.2''FWHM) which allows us to
  spatially resolve more complex structures, but lower sensitivity to
  extended sources given the higher surface brightness detection
  threshold.

Which implies that the AO are causing lower sensitivity. Maybe the lower sensitivity has something to do with the brightness of the laser used in the AO observations?